1. Field of the Invention
The present invention relates to a reading and writing method for a memory, and more particularly to a reading and writing method for a non-volatile memory with more than two data states.
2. Description of the Related Art
Portable electronic devices, such as cell phones, digital cameras and PDAs, electronic device are typically in standby mode or sleep mode more than in an operating mode. Non-volatile memory is required to ensure that data stored in an electronic device can be preserved without power. Currently memory cells of memory devices store only two data states. If the portable electronic device requires greater memory capacity, the only way to increase capacity is to increase the number of memory cells. In other words, the memory matrix of the memory device increases, thus, the size of the memory device increases. In order to increase the memory capacity and avoid increasing the volume of memory, a memory device with multiple data states is required. Currently, memory devices with multiple data states are achieved by the different resistance of the memory cells and if the resistance variation range is not great enough, the data state may be incorrectly determined. Magnetoresistive random access memory (MRAM), ovonic universal memory (OUM), resistive random access memory (RRAM) are memory devices with multiple data states generating the most interest. FIG. 1 is a schematic diagram of a memory cell of RRAM. Memory cell 10 comprises transistor T1 and metal-insulator-metal (MIM) element 11. The resistance of the MIM element 11 can be changed by applying a bias voltage via the node N. When the memory cell 10 is read by a reading voltage applying to the MIM element, the data state of data stored in the memory cell 10 is determined by current passing through the MIM element. However, if the resistance variation range is not great enough, the current difference between data states is not easily determined, thus, the data states may be incorrectly determined.